1. Field of the Invention
The present invention relates to a shading correction circuit of an electronic camera.
2. Description of the Related Art
Images shot by a solid image pickup device of an electronic camera have local distortion (shading) in amplitude owing to unevenness in illumination from light sources, in transmission of lenses, or sensitivities of sensors, etc. Japanese Unexamined Patent Application Publication No. Sho 63-299575 has been known for a correcting method of such shading. According to this correction method, shading is performed by capturing a uniform luminance surface, storing output values in a memory as correction coefficients for respective pixels, and using the correction coefficients at the time of outputting images that contain a subject.
The shading correction according to Japanese Unexamined Patent Application Publication No. Sho 63-299575 is, however, a complicated procedure because it requires a specific operation for obtaining correction coefficients other than the original operations for obtaining images. Besides, the correction coefficients can vary with environmental changes such as secular degradation of the system. Thus, in order to maintain the precision of the shading correction, the method disclosed therein requires periodical obtainment of the correction coefficients even after finishing reading them once, so that it has to be modified on this point.
In addition, the shading correction method disclosed therein requires a memory for retaining the correction coefficient for each pixel. This is disadvantageous in terms of cost performance since in particular, memories of a larger capacity has been demanded for retaining the correction coefficients recently due to soaring increases in the number of pixels constituting a single image.